U.S. Pat. No. 3,246,641 to Goehring granted Apr. 19, 1966 and assigned to the assignee of the present invention discloses a combination engine protective device operable to stop operation of an internal combustion engine upon the happening of certain abnormal conditions such as, for example, excessive crankcase or air box pressure or inadequate water pressure in the engine cooling system. The device is operative upon occurrence of any of these abnormal operating conditions to open a valve so as to drain oil from a pressure line connected with the engine governor. The pressure drop in the oil line in turn actuates means in the engine governor to stop operation of the engine.
The water-air box pressure portion of the protective device comprises a latching member engaged by a pair of diaphragms, one of which is responsive to pressure in the engine cooling system to urge the member toward its "latch" position and the other of which is responsive to pressure in the engine air box to urge the member toward its "release" position. In normal engine operation, the pressures in both the cooling system and the air box vary as functions of engine speed and so tend to offset one another. If, however, the engine water pressure is reduced or the air box pressure is increased an abnormal amount, the latching member is moved to its "release" position, tripping the protection device and stopping the engine.
The crankcase pressure portion of the protective device comprises a second latching member engaged by a single diaphragm responsive to pressure in the crankcase and to atmospheric pressure. The crankcase pressure is effective when it reaches an abnormal amount to urge the latching member to its release position, tripping the protective device and stopping the engine.
U.S. Pat. No. 3,958,548 to Koci et al granted May 25, 1976 and assigned to the assignee of the present invention discloses an improved engine protective device which refined the earlier invention and which, in lieu of the water pressure, uses the water pump inlet and outlet pressures applied to opposite sides of a diaphragm to arrive at a difference of those pressures which then is offset by the air box pressure to determine whether an abnormal pressure condition is present, and releases the latching member accordingly.
In each of those patents, the latching members hold the respective release valves in closed position but when released a spring bias opens the valves. Each of the latching members, when released, can be reset when the pressure condition returns to normal by pushing in a manually operated reset button attached to the valve to return the valve to closed position where the latch engages. While the devices of both the above-cited patents have been satisfactorily operated for many years, it has been found that a side effect of that construction is that, in an attempt to override the engine protector, the reset button might be held in to prevent valve opening when the "release" condition occurs.